Closed system neutron generator tube

ABSTRACT

A closed system, sealed-off neutron generator tube is described which generates 14 MeV neutrons from the T(d,n)He4 nuclear reaction on a deuterium and tritium absorption loaded target surface onto which accelerated deuterium and tritium gas ions are impinging. The electrode structure of the tube consists of a magnetic poleface of one polarity, a cathode electrode in front of and adjacent to, but electrically insulated from that poleface and, a non-magnetic metallic anode discharge chamber surrounding the cathode electrode in vacuum tight connection with the first magnetic poleface which chamber ends in a full-width opened emission aperture, the rim of which constitutes another magnetic poleface. An acceleration electrode aperture whose cross-section corresponds and is adjacent to the emission aperture constitutes a short and wide acceleration gap. A target electrode opposite to the cathode electrode beyond the acceleration gap is electrically connected to the acceleration electrode and its surface is prepared with a tritium and deuterium absorbed target sheath. A magnetic field of about 1,000 - 2,000 Oe is applied by outside current windings. The path of the magnetic field emerges from the poleface behind the cathode electrode, intersects the latter, spreads along the anode discharge chamber and declines laterally in the region of the emission aperture so as to enter the poleface at its rim. A magnetic yoke outside of the windings serves to close the magnetic path between the polefaces of either polarity. The target and acceleration electrode potential is maintained at about 100 - 300 kV, while the potential of the cathode electrode is maintained at about 10 - 30 kV, both potentials being applied with a negative polarity with respect to the anode discharge chamber by means of appropriate feed-through insulators. Forced water flow cooling is provided for the electrodes, part of the vacuum envelope and the magnetic coils. Positive ions are generated inside the anode discharge chamber by means of a self-sustained low pressure high-voltage cold cathode discharge with the ionizing electrons trapped in a region of crossed magnetic and electric fields. An electron cloud rotating differentially and of nearly uniform density thus ionizes the low pressure tritium deuterium gas mixture. The ions discharged from this region by the electric discharge field are either lost to the cathode electrode or to a larger degree driven in the opposite direction into the region of the wide emission aperture and enter the continuing electric field of the acceleration gap upon passing the acceleration electrode aperture, these ions obtain a sufficient velocity to release neutrons from the fusion reactions when they finally impinge on the target surface material. Secondary electrons, released from the target surface by the impinging ions and accelerated in the opposite direction to the ion flow, pass the acceleration gap and the discharge region and must be intercepted by the cathode electrode. High flux neutron irradiation of samples for activation analysis or production of short-lived radioisotopes is achieved inside a cylindrical target. In this geometry, a fairly uniform neutron flux distribution is produced by bombarding the cylindrical target at its outside with ions generated in an annular discharge region concentric to the target, whicH ions are accelerated radially inward onto the target. The axially measured width of the ion current density distribution should be comparable to the diameter of the target cylinder. A well collimated neutron beam, with a high dose rate applicable in medical cancer therapy irradiation with fast neutrons, is produced in the axial direction to the target cylinder outside of a thick radiation shield which is equipped with a collimating bore with the cross-section corresponding to that of the target cylinder. Optimum collimation is obtained by a slightly conical target cylinder having the form of a truncated cone with its apex located half-way on the axis along the collimating bore. An inside shielding cone resembling the residual of the target frustrum up to the apex tip is inserted into the first half of the collimating bore. The latter may be interchangable, thus being either divergent, cylindrical or convergent, depending on the size of the irradiation field to be actually used.

[ 1 Jan. 15, I974 CLOSED SYSTEM NEUTRON GENERATOR TUBE [75] Inventor: Albrecht Schmidt, Karlsruhe,

Germany [73] Assignee: Gesellschaft fur Keinforschung mbH, Karlsruhe, Germany [22] Filed: Feb. 1, 1972 [21] Appl. No.: 222,639

3,371,238 2/1968 Beckurts et al..... 250/845 X 3,417,245 12/1968 Schmidt 250/845 3,609,369 9/1971 Croitoru 250/845 Primary Examiner-Archie R. Borchelt A ttorney- Spencer and Kaye [57] ABSTRACT A closed system, sealed-off neutron generator tube is described which generates 14 MeV neutrons from the T(d,n)l-le nuclear reaction on a deuterium and tritium absorption loaded target surface onto which accelerated deuterium and tritium gas ions are impinging..

The electrode structure of the tube consists of a magnetic poleface of one polarity, a cathode electrode in front of and adjacent to, but electrically insulated from that poleface and, a non-magnetic metallic anode discharge chamber surrounding the cathode electrode in vacuum tight connection with the first magnetic poleface which chamber ends in a full-width opened emission aperture, the rim of which constitutes another magnetic poleface. An acceleration electrode aperture whose cross-section corresponds and is adjacent to the emission aperture constitutes a short and wide acceleration gap. A target electrode opposite to the cathode electrode beyond the acceleration gap is electrically connected to the acceleration electrode and its surface is prepared with a tritium and deuterium absorbed target sheath. A magnetic field of about 1,000 2,000 Oe is applied by outside current windings. The path of the magnetic field emerges from the poleface behind the cathode electrode, intersects the latter, spreads along the anode discharge chamber and declines laterally in the region of the emission aperture so as to enter the poleface at its rim. A magnetic yoke outside of the windings serves to close the magnetic path between the polefaces of either polarity.

The target and acceleration electrode potential is maintained at about 100 300 kV, while the potential of the cathode electrode is maintained at about 10 30 kV, both potentials being applied with a negative polarity with respect to the anode discharge chamber by means of appropriate feed-through insulators. Forced water flow cooling is provided for the electrodes, part of the vacuum envelope and the magnetic coils.

Positive ions are generated inside the anode discharge chamber by means of a self-sustained low pressure high-voltage cold cathode discharge with the ionizing electrons trapped in a region of crossed magnetic and electric fields. An electron cloud rotating differentially and of nearly uniform density thus ionizes the low pressure tritium deuterium gas mixture. The ions discharged from this region by the electric discharge field are either lost to the cathode electrode or to a larger degree driven in the opposite direction into the region of the wide emission aperture and enter the continuing electric field of the acceleration gap upon passing the acceleration electrode aperture, these ions obtain a sufficient velocity to release neutrons from the fusion reactions when they finally impinge on the target surface material.

Secondary electrons, released from the target surface by the impinging ions and accelerated in the opposite direction to the ion flow, pass the acceleration gap and the discharge region and must be intercepted by the cathode electrode.

High flux neutron irradiation of samples for activation analysis or production of short-lived radioisotopes is achieved inside a cylindrical target. In this geometry, a fairly uniform neutron flux distribution is produced by bombarding the cylindrical target at its outside with ions generated in an annular discharge region concentric to the target, which ions are accelerated radially inward onto the target. The axially measured width of the ion current density distribution should be comparable to the diameter of the target cylinder.

A well collimated neutron beam, with a high dose rate applicable in medical cancer therapy irradiation with fast neutrons, is produced in the axial direction to the target cylinder outside of a thick radiation shield which is equipped with a collimating bore with the cross-section corresponding to that of the target cylinder. Optimum collimation is obtained by a slightly conical target cylinder having the form of a truncated cone with its apex located half-way on the axis along the collimating bore. An inside shielding cone resembling the residual of the target frustrum up to the apex tip is inserted into the first half of the collimating bore. The latter may be interchangable, thus being either divergent, cylindrical or convergent, depending on the size of the irradiation field to be actually used.

15 Claims, 9 Drawing Figures PATENTEBJAH 15 1914 saw 1. or 6 :ATENTEDJM 151974 I 3; 786, 258

' SHEET 3 OF 6 PATENTEDMH 1 SW 3.786; 258

SHEET u 0F 6 RABBIT SYSTEM Fig.6

VOLTAGE Z09 SUPPLY CONTROLLED CURRENT LOW VOLTAGE SUPPLY SUPPLY CONTROL UNIT HIGH 770 VOLTAGE SUPPLY 711 VOLTAGE STABILIZER PRESSURE GAUGE CLOSED SYSTEM NEUTRON GENERATOR TUBE BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a neutron generator with a sealed-off low pressure high-voltage discharge system in which ions are generated in one partial volume (discharge space) with crossed magnetic and electric fields, which ions are accelerated at the same pressure in a different, immediately adjacent partial volume (acceleration space) onto a target electrode with a high negative potential and generate neutrons by means of nuclear reactions in a target material attached to the target electrode.

Neutron generators with sealed-off tubes use Penning ion sources, high frequency ion sources or non selfsustained low pressure discharge systems with hot cathodes. In these systems, the ionization probability can be enhanced by increasing the path of the elctrons by means of magnetic fields. Such sealed-off neutron generators reach maximum neutron source strengths of 10 n/sec at acceleration voltages of approximately 150 kV with ion currents consisting of equal fractions of deuterium and tritium ions on the maximum order of 10 mA.

The principle of sealed-off neutron generators, which are not operated with a pump but use a constant filling of deuterium gas D and tritium gas T contrary to conventional d-t neutron generators, allows a constant neutron yield to be obtained over a very long period of time, because the tritium content of the target is not diluted by the impinging deuterons but is constantly renewed by injected tritium ions, i.e., it remains in a steady state with respect to the concentration of the target nuclei.

The life of the target is determined only by the very low sputtering rate of the titanium hydride layer of the target under impingement of the heavy hydrogen isotopes and amounts to some 1,000 hours at full load and target current densities of approximately 1 mA/cm 2. Description of the Prior Art In a familiar process, Pennion ion sources are used to generate ion currents in sealed-off neutron generators in which the sources are crossed, preferably radial electric and preferably axial magnetic fields are used in cylindrically symmetric systems with axial ion extraction.

In these designs, a hollow cylindrical anode is located in a coaxial magnetic field. The inner space of the anode is closed in the axial direction by two cathode electrodes. One of the cathode electrodes contains an emission aperture for emission of part of the ions generated in the space surrounded by the anode (discharge space). Acceleration electrodes installed in front of the emission aperture generate an electric high voltage field for acceleration of the ions in the direction of the target which is arranged normal to the axis of the anode. The gas stream emitted as an ion beam by the ion source must be replaced during the steady-state mode by an equal quantity of neutral gas flowing back through the emission aperture. This limits the maximum emission current which is obtainable in a system with a preset gas pressure. Because of the required high voltage stability of the acceleration system and because of the beam current and energy losses, respectively, of the particles along the beam path, which greatly increase with the gas pressure, it is generally not possible to increase the emission current by increasing the gas pressure.

In another known embodiment, a hollow cylindrical anode is installed between two target electrodes operated at a high negative potential one of which carries a target. In this embodiment a magnetic field, which preferably is in the axial direction is generated in the interior of the anode by permanent magnetic rings. In each of the two space regions between the target electrodes and the anode cylinder, there is an electric field along with the external magnetic field, which is generated by the permanent magnets. Within the anode cylinder there is a radial electric space charge field induced by the discharge together with the axial inner magnetic field. Hence, there are three areas with crossed electric and magnetic fields in which electron clouds may be captured for the generation of positive ions which are accelerated onto the target electrode.

Operation of this system requires a relatively high gas pressure 9 X 10 Torr) because otherwise there will be no discharge. As a consequence of the high gas pressure, most of the ions are generated in the region between the anode cylinder and the target electrodes so that the ions are generated at a variety of different potentials and a very large part of the ion current, because of its low energy, does not contribute to neutron generation at the target, which results in only low neutron yields. The ions coming' from the inner space of the anode cylinder, which drop through approximately the whole potential, only constitute a small fraction of the ion current reaching the target and are greatly attenuated by charge transfer collisions at the relatively high neutral gas density. Ions hitting the other target electrode not covered with a target layer form a deuterium self-target there during operation, thereby generating parasitic neutron radiation which impedes the use of the device.

Moreover, a concept is known under which a tubular target is bombarded with ions by a multitude of conventional ion sources arranged on the periphery of an annular acceleration space or by one or more familiar annular ion sources using concentric acceleration electrodes.

in plasma and fusion research, annular ion sources of sometimes considerable dimensions (diameters up to 2.5 m) are also utilized. They are designed, e.g., as magnetron ion sources or annular Penning ion sources for beam currents up to 1 ampere. However, such devices can be operated only with vacuum pumps of a high pumping power and require a major pressure gradient between the ion source and the post-acceleration space, with a high gas stream passing through a narrow emission aperture designed as an annular slot. Because of the relatively high operating pressure and the strong throttling of the stream of neutral gas flowing back from the target to the discharge space, these annular ion sources, just as the familiar Penning ion sources, are not suitable for utilization in sealed-off neutron generators for high beam currents.

The present invention is based on the objective of creating a sealed-off neutron generator which will produce a high neutron flux of great homogeneity in a predetermined volume and will operate over a long life span with a high degree of reliability and which can be used for specific applications, e.g., in activation analysis, nuclear safeguards control, the production of short- SUMMARY OF THE INVENTION In the present invention, this problem is solved in that the ions are generated in the interior of a hollow metal I body (anode electrode) operated at an anode potential and completely open to the target electrode and that there is a cathode electrode on the reverse side removed from the target electrode in the interior of the hollow body by a small distance which almost completely covers this side, that the aperture of the hollow body (emission aperture) facing the acceleration space and the target electrode has a rim designed as an annular magnetic pole surface of one polarity and the side of the hollow body covered by the cathode is designed as a magnetic pole surface of the other polarity, and that the hollow body is surrounded by means of excitation of a magnetic flux emanating from the magnetic pole surface on the cathode side, which flux permeates the cathode, the discharge space, and the region of the emission aperture and is taken up by the annular magnetic pole surface on the rim of the emission aperture.

It has been found that the inclusion of electrons in a discharge space does not necessarily require the use of a material second cathode with an emission hole facing the cathode. The function of this second cathode can be produced also by an immaterial equipotential surface having the same potential as the first cathode. On the other hand, this equipotential surface allows ions generated in the discharge space to move freely into the post-acceleration space, thus avoiding the dissipation losses which would occur there in the presence of this electrode.

A diaphragm-type acceleration electrode, whose aperture corresponds to the size of the emission aperture, is installed between the emission aperture and the target electrode and operated near the potential of the target electrode.

As a means of excitation of the magnetic flux, an electromagnet is installed enclosing the discharge space outside the vacuum container. The yoke of the electromagnet connects the magnetic pole surface on the cathode side and the magnetic pole surface on the rim of the emission aperture of the anode electrode facing the target electrode and includes the exciting coils.

In this design, it has turned out to be advantageous to design the hollow metal body and/or the magnetic pole surface on the cathode side, the magnetic pole surfaces on the rim of the emission aperture, and parts of the yoke of the magnet facing the acceleration space as part of the vacuum container of the discharge space and the acceleration space.

In a special embodiment of the neutron generator according to the present invention, the discharge space in which the ions are generated is constructed in an annular form (annular discharge space) and an aperture, also annular, is provided on the inside of that space which is oriented towards the axis of the annular discharge space (emission aperture) the ions escape from the annular discharge space through the emission aperture into an acceleration space (also annular) and are accelerated by a radial, convergent electric field generated by electrodes arranged there onto a target electrode of an essentially tubular shape, which target electrode is arranged coaxial to the annular discharge space.

The arrangement according to the invention of the target electrode in the center of an annular discharge space results in an ion current density on the target layer at a very favorable current density distribution which is higher than the ion current density in the emission aperture. The radially symmetric acceleration field causes geometric focusing of the ion current in the radial direction, while the divergence of the ion current in the axial direction existing at the outlet from the emission aperture, is influenced only slightly so that the section of the target electrode hit by the ion current can be made larger in its axial extension that the width of the emission aperture.

High neutron fluxes of good homogeneity at high neutron emission current densities are achieved especially if the target as the surface source of fast neutrons (14 MeV) encloses the sample to be irradiated on all sides like a cylinder. These characteristics are preconditions to the use of the neutron generator in certain applications, such as activation analysis, nuclear safeguards control, and generation of short-lived radionuclides, in which very high homogeneous neutron fluxes in a predetermined volume are required.

A particularly simple design is achieved for the neutron generator according to the present invention in that the outer periphery of an annular discharge space carries an annular cathode (cathode ring) and the discharge space is limited in the direction of the axis of the cathode ring by two annular disk shaped anodes (ring anodes) installed on both sides of the cathode ring parallel to the plane of the cathode ring at a distance corresponding to the axial length of the cathode ring.

In the interior of the annular discharge space, a mainly radial magnetic field is generated by two magnet coils arranged outside the vacuum container concentric with the axis of the cathode ring in the area of the annular anode. Both of the magnet coils are enclosed by a magnetic yoke for guidance of the magnetic flux. On the inside of the magnetic yoke facing the axis of the cathode ring there is an annular gap acting as the emission aperture for the ions, the width of this gap corresponds to the distance between the annular anodes.

Between the annular gap of the magnetic yoke serving for ion emission and the tubular target electrode, two plate-shaped acceleration electrodes, which are operated close to the potential of the target electrode, are arranged on both sides of the annular gap so that their axial distance, starting from the width of the annular gap, increases with decreasing radius up to the axial width of the ion current impinging upon the target. The electrodes and/or the vacuum housing are preferably equipped with cooling channels for the passage of a coolant.

In the axial direction, a high voltage insulator is connected to one side of the annular ion source so as to be vacuum tight and the tubular target electrode is carried through the center of the annular ion source and the insulator and attached to the outer front face of the latter so as to be vacuum tight and in such a way that the inner space of the tubular target is accessible from the outside for loading the specimens to be irradiated.

The coils generating the magnetic field are encased in a water tight shell of corrosion-resistant material and surrounded by a magnetic yoke and the anode, which constitutes one unit with the magnetic yoke, in such a way that a cooling channel is formed for passing through a coolant which cools the coils and the vacuum container. The cathode ring is made of metal tubes whose windings are welded or soldered to each other. A coolant is fed through these tubes.

In a preferred embodiment of the present invention, the target electrode is made essentially of a metal tube supporting the structure (support tube) and a metal tube wound upon the support tube in a bifilar arrangement in the region of the ion current for transporting a coolant, the windings of the coolant line are weldedto each other and the target layer is produced on the surface thus formed.

In certain applications it can be preferable to close the support tube on the target side and install a coaxial guide tube connected with a rabbit system in the support tube for carrying an irradiation sample so that it is centered at the closure of the supporting tube and provided with penetrations at this end. In order to recover a sample after irradiation, gas is fed into the annulus between the support tube and the guide tube so as to enter the guide tube through the penetrations. It is possible also to recover the sample by generating a vacuum.

A high voltage insulator is attached to the front of the annular ion source, in the axial direction so as to be vacuum tight and the support tube is carried through the center of the annular ion source and the insulator and attached to the outer front of the latter, also in such a manner as to be vacuum tight. The support tube is extended beyond the point of attachment and equipped with a connection for the introduction of gas into the annular channel between the support tube and the guide tube.

In special applications, such as the production of radionuclides or for purposes of nuclear safeguards control, it may be preferable not to close the guide tube for the accommodation of substances to be irradiated at the end of the target electrode but carry it through the high vacuum system of the neutron generator and attach it to two insulators arranged axially on both sides of the target electrode so as to generate a through channel for irradiation. Instead of the bifilar coolant tube being wound on the support tube, a singly wound cooling tube may be used. It is possible also to design the support tube as a target electrode in a specific area and feed coolant through the annular channel formed between the support tube and the guide tube.

Other features provided include a capsule for accommmodation of a sample to be irradiated with neutrons, means of introducing the capsule into the guide tube, and means of fixing the capsule in the part of the guide tube enclosed by the target electrode for a preset period of time for irradiation with neutrons.

The application of fast neutrons in radiotherapy requires particularly high effective neutron emission current densities with extremely high overall source strengths so the intensity of neutron radiation existing at a distance of approximately 1 m from the source is sufficient to generate a neutron dose rate in the irradiation field on the order of 1,000 rem within ten minutes. This requires the distance of the patient from the neutron source to be sufficient for for collimation of the neutron radiation and for shielding of those parts of the body which must not be irradiated.

Application of the neutron generator according to the invention in radiotherapy with fast neutrons is possible in a very simple way by a modified design. In this modification, the tubular target electrode is designed as a weakly conical frustrum at its closed end in the area of the target layer. This conical frustrum is part of a cone whose tip is located outside the vacuum housing.

On the side facing away from the high voltage insula: tor, means of collimation of the neutron flux emitted by the target layer are attached to the vacuum housing which consist essentially of a supplementary cone (collimation cone) of the conical frustrum constituted by the target, and of a block with a conical or cylindrical through bore (collimation channel), the collimation cone extending into the inlet section of the collimation channel, which makes the cross section of the latter annular in this region.

BRIEF DESCRIPTIONOF THE DRAWINGS FIG. 1 is a schematic representation of the ion source and the acceleration system.

' FIG. 2 shows the potential curves in the direction of the axis of the ion beam and perpendicular to it.

FIG. 3 shows the ion current density on the target electrode.

FIG. 4 shows the neutron generator with an annular ion source and a tubular target.

FIG. 5 is a section AA through the neutron generator shown in FIG. 4.

FIG. 6 is a schematic representation of the voltage and power supply systems.

FIG. 7 is a schematic representation of the electrode cooling system.

FIG. 8 is a simplified cross section through a conical target electrode.

FIG. 9 shows a neutron generator with a weakly conical target in a radiotherapy device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic representation of a possible embodiment of the discharge and acceleration system according to the invention. A discharge space is partly enclosed by a tubular, nonmagnetic anode electrode 1 and a pole surface 8 made of magnetic material closing the anode electrode on one side. In the discharge space, parallel to the pole surface 8, and a small distance away from the latter, a cathode electrode 2 is arranged. A target electrode 3 (target) is set up opposite the cathode electrode 2 so that the axis of symmetry 4 of the anode electrode, which is the axis of the ion beam, connects the cathode with the target and is essentially perpendicular on both electrodes. Also symmetric to the axis of the ion beam 4, an acceleration electrode 5 of the shape of a lateral area of a cone is arranged on target 3 which limits the acceleration space. The anode electrode 1 is enclosed by the exciting coil 6 which, in turn, is enclosed by a magnet yoke 7 with the pole surfaces 8 and 9.

The cathode electrode 2 is operated at a negative high voltage potential of the voltage --V, relative to the anode electrode 1. The target 3 and the acceleration electrode 5 are kept at a much higher negative potential V,. This potential distribution results in the curve of the electric field indicated in dashed lines, there being in the region of the emission aperture enclosed by the pole surface 9 an equipotential surface 10 which is at the potential of the cathode 2 and separates the discharge space from the acceleration space in the direction of the target.

Between the pole surface 8 and 9 there is a magnetic field extending mainly in the direction of the axis of symmetry 4 whose field lines (dash-dotted lines) deviate from the axis of symmetry with increasing distance from it and are bent towards the pole surface 9 in the region of that pole surface 9. The family of field lines 11 which is barely touched by the equipotential surface in the region of the emission aperture constitutes a magnetic field surface subdividing the discharge space into an inner region 12 (ionization space) enclosed by this magnetic field surface and an outer region located between this magnetic field surface and the anode.

In the ionization space 12, all magnetic field lines coming from the cathode 2 run so as to intersect the equipotential surface 10. The magnetic field lines extending outside the ionization space 12 and the magnetic field surface 11 do not reach the equipotential surface 10 but intersect the anode electrode 1 or directly converge into the pole surface 9.

FIG. 2 shows the potential curve 13 along the axis of symmetry 4 extending between the cathode 2 and the target 3 and another curve 14 in a sectional plane AA perpendicular to it. Both potentials have an essentially parabolic shape in the ionization space 12. At the point of intersection K of the axis of symmetry 4 with the equipotential surface 10, the potential is equal to the cathode potential K and then drops very sharply to the target potential V in the acceleration space 15 limited by the acceleration electrodes 5.

An electron released from the cathode, e.g., as a consequence of an ion impact, is accelerated in the direction of the axis of symmetry 4 and delayed correspondingly beyond the center plane AA and reflected towards the cathode at the equipotential surface 10 near point K. If the electron loses energy on this path, e.g., as a result of an ionizing collision, it will not reach the cathode again. Instead, such an electron will be retained in the vicinity of its marginal field surface so as to be captured in the ionization space 12. A multitude of electrons captured build up an electron cloud of an almost homogeneous space charge in the ionization space 12. Under the combined effects of the electric and magnetic fields, the electrons of this cloud drift perpendicularly with respect to these fields in the ionization space 12 in such a way as to define narrow cycloid tracks near their field areas. Only collisions transmitting energy can cause such an electron to change from one magnetic field surface to a field surface closer to the anode. The positive ions generated in this way by ionization processes have an oscillating potential corresponding to curve 14 and leave the ionization space simultaneously in the direction of the axis of symmetry towards the cathode 2 and the equipotential surface 10. Ions permeating the equipotential surface 10 are accelerated onto the target in the acceleration space 15 by the electric field existing between the pole surface 9 and the acceleration electrode 5. All ions produced in the part of the ionization space situated on the target side of the AA plane in this way reach the target 3. The ions generated on the cathode side of the AA plane fall onto the cathode 2 with a much lower energy.

The resultant current density distribution of the ions on the surface of the target roughly corresponds to the curve shown in FIG. 3 with the maximum value in the axis of symmetry 4 being approximately equal to twice the mean value of the distribution.

At an energy of approximately keV, the ions impinging upon the target release about 4 secondary electrons per impinging ion in the titanium layer saturated with equal fractions of deuterium and tritium, which secondary electrons drop through the acceleration field in the reverse direction as initially slow electrons. They travel through the ionization space 12 as fast electrons, are kept away from the anode 1 by the magnetic field and impinge upon the cathode 2 where they release their energy. Because of their high energy, these fast electrons participate in ionization only to an insignificant extent. The secondary electron fraction streaming back from the target can be greatly reduced by means of a small positive inverse voltage imposed upon the target relative to the acceleration electrode.

Of course, it is possible also to use plane electrodes instead of the tubular anode 1 and the acceleration electrode 5 of the shape of the lateral area of a cone.

If the sectional view of the assembly shown in FIG. 1 is made to rotate around an axis 16 in the plane of the drawing, this results in a rotationally symmetric system with an annular arrangement of the ion source discharge space and a coaxial tubular target on the surface of which an increased ion current density is achieved because of the geometry now existing.

FIGS. 4 and 5 show such a neutron generator with an annular ion source and a tubular target.

Two anodes 17 of the shape of annular disks and made of stainless steel or any other non-magnetic material are welded onto a ferromagnetic pole shoe ring 18 in such a way as to generate an annular chamber open on the side facing towards the center.

The cathode ring 19 consists of four rings of copper tube welded onto each other. A connection piece 20 connects the four tubes in parallel and accommodates the coolant feed line 21 and the coolant discharge line 22 which are run through one bore each in the pole shoe ring 18 and connected with the pole shoe ring through one ceramic feed-through insulator 23 each so as to be vacuum tight. The connection piece 20 has cooling channels 24.

A threaded bushing 26 is welded into each of two other bores 25 in the pole shoe ring 18 distributed on the periphery with a ceramic support pin 28 put through one guide element 27 each in the center of these bushings. Each of the two support pins 28 is engaged in a bushing 29 welded onto the cathode ring 19 and is elastically supported in this position by a spring 30 and a bolt 31.

Each of the magnet coils 32 and 33 is enclosed on all sides by a housing 34 made of stainless steel and pushed over the ferromagnetic cylinders 35 and 36, respectively, which are welded onto one each of the anode rings 17 with their fronts 37 acting as pole shoes. The ring 38 and the cylinder 39 are welded together at right angles and connected with the cylinder 35 by screw joints 40 distributed over the periphery with seals 41 and 42 so as to be liquid-tight. In a corresponding way, the ring 43 and the cylinder 44 are connected with each other and with the cylinder 36. Clamps 45 distributed over the periphery keep together the cylinders 39 and 44 and ensure good magnetic contact with the pole shoe ring 18. In this way, the coil 32 is enclosed by a magnet yoke consisting of the components 35, 38 and 39 with the pole shoes 18 and 37, and the coil 33 is enclosed by a corresponding magnetic yoke consisting of the components 36, 43 and 44 with the pole shoes 18 and 37.

The housing 34 of the magnet coils 32 and 33 are retained in their annular chambers formed by the magnet yokes and the anode rings 17 by means of spacers not shown in such a way that a coolant fed in through the inlet pipe 46 flows around the coil from all sides in an annular channel 47, thus cooling the coil, anode ring and magnet yoke. The annular channel 47, which is sealed relative to the separation line of the pats 39 and 44 of the magnet yoke by a seal 48 and one other seal each 49 between the housing 34 and the parts 39 and 44, respectively, of the magnet yoke, can provide im proved cooling if the inlet pipe 46 and the outlet pipe 50 positioned diametric to it are arranged on different sides of the ring seal 49.

The cylinder 35 of the magnet yoke serving at the same time as a vacuum container is closed vacuum tight by a welded-on lid 51 on its side facing away from the pole shoe 37.

In the center of the annular discharge space partly enclosed by the anodes l7 and the cathode ring 19, there is a support tube 52 the lower end of which is closed vacuum tight by means of a lid 53 which is welded on. A copper tube 54 is wound on the support tube 52 in a bifilar windings, the tube coils are welded together and a target layer is put onto the surface of the target electrode thus generated. Plate-shaped acceleration electrodes 55 made, e.g., of copper are attached by welding to the upper and lower ends of the target electrode and tubes 56 are welded onto the edges of these acceleration electrodes adjacent to the pole shoe rings 37 to reduce the electric field strength, the axial distance of which tubes increases to the length of the target layer with decreasing radius.

The support tube 52, the inlet line 57 and the discharge line 58 of the coolant of the target electrode are welded into bores of a lid 59 which is connected with a ring 62 so as to be vacuum tight through bolted connections 60 and a metal seal 61. The ring 62 is welded onto the metal end ring 63 of an insulation tube 64 whose other end ring 63 is welded onto a conical ring 65 which, in turn, is attached to the cylinder 36 of the magnet yoke by a welding connection so as to be vacuum tight.

The support tube 52 and the inlet line 57 and the discharge line 58 are guided in another lid 66 through bores further up where they are sealed by means of rubber seals 67 and 68 and a contact plate 70 retained by bolts 69. The lid 66 is connected with the head of a high voltage insulator through a seal 71 via bolted connections 72, a split ring 73, and a pressure equalization ring 74 so as to be oil tight. The foot of the high voltage insulator 75 is connected with the ring 43 of the magnet yoke through a split clamping ring 76, a rubber seal 77, a pressure equalization ring 78, and bolted connections 79 so as to be oiltight. The space 80 between the high voltage insulator 75, the conical ring 65, the insulator 64, the lid 62, the support tube 52, and the lid 66 is filled with an insulating oil or insulating gas.

A guide tube 81 made of aluminum or stainless steel is coaxially arranged in the support tube 52 and welded onto a threaded bushing 82. The support tube and the guide tube are connected with each other by a locking notch with a c-shaped wire returning ring 83 and a compression nut 84 arranged at the head of the support tube and are sealed with seals 85 and 86. Compressed air can be supplied to the annulus between the support tube 52 and the guide tube 81 via the inlet pipe 87, which compressed air enters the guide tube through bores 88 at the lower end of the guide tube, ejecting an irradiation capsule 89 from the neutron generator. A hose 90 made of insulating material is connected with the upper end of the guide tube which serves for connection, e.g., with a rabbit system 139 (see FIG. 6).

For appropriate potential distribution, a split aluminum sphere 91 is connected with the split ring 73 which sphere surrounds the connecting pipes for the coolant 57, 58 compressed air 97, and the endof the guide tube 81. The line 90 of the rabbit system is guided into the aluminum sphere 91 through an opening 92, the compressed air line 93 and the coolant lines 94 and 95 pass through an opening 96.

A tube section 98 with a conical seal 99 is connected to each of the two threaded bushings 26 by means of a cap screw 97 so as to be vacuum tight. An evacuation line 100 made of copper is welded onto one of the tube sections 94 via an intermediate ring, which line is cold welded by squeezing after evacuation to separate the neutron generator from the vacuum pump.

For measurement of the pressure during filling of the discharge gas (e.g., deuterium tritium) into the sealed-off system, this tube section 98 is equipped with a small NTC resistor 101 electrically connected through a ceramic penetration insulator 102. A housing 103 is connected to the other tube section 98 in which a gas pressure regulator 104 according to the German Pat. No. 1,275,691 and a helical nickel tube 105 (nickel valve) closed on one side and heated by direct passage of current are installed. The discharge gas is fed into the nickel tube 105 for filling into the sealedoff system and then difi'uses through the wall of the latter into the vacuum space if the heater is turned on.

During filling and absorption, the pressure in the vacuum space is measured with the NTC resistor 101 operated as a thermal conductivity gauge. After filling of a predetermined quantity of gas the heater of the nickel tube is shut off and filling is terminated. Most of the predetermined quantity of discharge gas so introduced into the system is spontaneously absorbed in the target layer (e.g., titanium) attached to the target electrode 54 close to saturation. Afterwards, the rest of the discharge gas filling is accomodated by the gas pressure regulator 104 and is used for setting and maintaining a preset gas pressure during operation.

The gas pressure regulator 104 consists essentially of a small Penning discharge system in the magnetic field of the magnet 106. Two titanium electrodes 107 arranged close to the axis of the discharge cell take up discharge gas and release it again in a controlled manner through an external control circuit upon electric heating of the wire anode ring 108. The gas pressure regulator allows the measurement of the gas pressure during operation, at the same time absorbing any undesired gases produced which, contrary to the discharge gas hydrogen, are not reversibly bound.

The block diagrams of FIGS. 6 and 7 are schematic representations of the main auxiliary equipment necessary for operation of the neutron generator.

The negative high voltage (approximately to 200 kV) for the target electrode 54 and the acceleration electrode 56 is generated by a high voltage power supply unit 109 and supplied through the aluminum sphere 91. The housing of the neutron generator and, in particular, the anodes 17 are operated at zero potential.

The cathode ring 19 is supplied with an adjustable negative voltage between and kV by a high volt age power supply 110. The fraction of the discharge current flowing from the cathode ring 19 is discharged against the zero potential by a voltage stabilizer 111 converting the power generated. Voltage stabilizers may be, for instance, arresters designed as voltage dependent resistors, Zener diode circuits, or a magnetron with an adjustable external magnetic field for control of the voltage.

The magnet coils 32 and 33 are series connected and are supplied from a current-controlled low voltage power supply 112.

The gas pressure regulation system 104 is supplied with high voltage in the range between 1 and 3 kV from the control unit 113. The discharge current occurring in the gas pressure regulation system 104 depends on the gas pressure and acts on the heater of the wire anode ring 108 via a controlling amplifier so as to reduce the deviation in control of the gas pressure.

Before the neutron generator is started up for the first time, the nickel valve 105 is heated with a controlled-current power supply 114 for filling the discharge gas. For control measurement of the gas pressure during absorption, an electronic thermal conductivity pressure gauge 115 is connected with the NTC resistor 101.

The electrode dissipation occurring during operation is removed by a cooling curcuit with a coolant of minimum electric conductivity, e.g., distilled water, which is recooled, e.g., with tap water in a heat exchanger 116. The water heated by the electrodes (anodes 17, cathode 19, target electrode 54) and the magnet coils 32 and 33 is removed by the lines 22, 50, and 58 flows through a degassing vessel 117 whose surface is blanketed with an inert gas. The coolant taken in by the pump 118 is recycled to the electrodes under pressure throughlines 21, 46, and 57 after cooling in the heat exchanger 116. A partial stream of the coolant is fed through an ion exchanger 119 to maintain the required low electrical conductivity. The cooling system is connected by insulating lines with the coolant connections of the electrodes operated at high voltage potential.

FIGS. 8 and 9 show a simplified representation of an embodiment of the neutron generator according to the invention for application in radiotheraphy with fast neutrons.

In this neutron generator, the tubular target electrode 120 is conical in the region of the target layer. A cooling channel 122 is constituted by a tube 121 coaxially inserted into the target electrode and also conical in the target area. The coolant is forced into the tube 121 through an insulating flexible line 123 and carried along the target tube 120 at high velocity through the annular channel 122.

Two plate-shaped acceleration electrodes 124 and 125 are installed. The electrode 125 is attached to the lower end of the target electrode 120. In the region of the axial projection of the lateral area of the cone of the target layer onto the central region of the electrode 125, this carries penetrations 126 around its periphery or has a reduced wall thickness. In this way, a possible attenuation of the neutron radiation emitted in this direction is minimized.

The same purpose is served by the reduction of the wall thickness of the lower end lid 127 of the housing of the neutron generator also in this annular area.

For collimation of the neutron radiation emitted in the direction of the axis of the target electrode there is an inner collimation cone 128 and a collimation tube 129 with a cylindrical or slightly conical bore. If the conicity of the cone 128 is approximately identical with that of the target electrode 120, the conditions of collimation are favorable. For the same reasons the diameter of the bore of the collimation tube 19 is approximately equal to the maximum diameter of the target layer attached to the target electrode 120.

The collimator consisting essentially of the components 128 and 129 and the neutron generator are inserted into a sphere 130 (shielding sphere), used for shielding against fast neutrons, in such a way as to position the target as the neutron source in the center of the sphere.

The shielding sphere may be made, e.g., of homogeneous steel or a suitable combination of steel and fiberboard or similar shielding materials for fast neutrons, e.g., also titanium hydride, zirconium hydride, lithium hydride, or concrete containing a large amount of water.

The composition of the shielding sphere 130 and its dimensions are chosen so that all parts of the body of the patient not to be subjected to radiotherapy are exposed only to such relatively low radiation doses during the period of radiation exposure as are biologically admissible.

The direct neutron radiation is collimated by the collimator. The collimator can be adapted to specific purposes by means of exchangeable collimation tubes 129 with different geometries of the bores.

The shielding sphere 130 is mounted in a semicardanic suspension with a suspension ring 131 and can be tilted around the axes 132 and 133. Parallel to the axis of the collimator, a telescopic rod system 134 is connected to the support ring 131 whose end points 135 carry a hinged swiveling structure 136 and on the rails 137 of which a couch 138 for a patient is installed so as to be movable. A control device not shown ensures that maximum efficiency of the collimated neutron beam in a predetermined volume of the body of the patient is achieved in oscillating exposures.

I Of course, the neutron generator according to the invention can be run also in the pulsed mode with higher instantaneous current densities than can be achieved in steady-state operation, under the condition that at a preset duty cycle of the sequence of pulses the average power loading capacity of the electrodes is not exceeded.

Special advantages of the neutron generator according to the invention are its surprisingly small dimensions relative to conventional neutron generators and the much higher powers achieved over considerably longer target lives. Familiar neutron generators with a sealed-off discharge system generate neutron source strengths which are lower by several orders of magnitude at comparable dimensions.

Moreover, the present invention allows the construction of large volume annular ion sources with favorable radial focusing of the ions onto a central target. At the same time, the axial distribution of the ion current on the surface of the target electrode can be set in a predetermined way through the choice of the geometry of the acceleration electrodes and, e.g., can be larger or smaller than the axial dimension of the ion generating system. In no place does the maximum ion current density exceed twice its average value so that, on the whole, a high average load is achieved and a high nondirectional neutron flux evenly distributed throughout a relatively large volume is generated in the inner space of the tubular target electrode. The generation of the radial ion beam is very effective with respect to power consumption in the annular ion source and allows the system to be operated at a relatively low pressure so that the ion current can also be accelerated with very small losses.

Moreover, the simple design of the system allows a high power capability at the target electrode. The related high fraction of secondary electrons does result in an increased power at the cathode, but the thermal energy this generated can be removed by the large area of the cathode ring without any problems.

Another advantage of the neutron generator according to the present invention results especially from its application in radiotherapy. In this application, the large area neutron source consisting of a conical target electrode is contracted to a very small effective neutron source area of an annular shape by projection of the lateral area of the cone in the axial direction, with a corresponding increase in the effective emission density of the neutron radiation. Only this allows favorable collimation of the neutron beam, largely avoiding halfshadow areas in the irradiation field.

What I claim is:

l. A neutron generator comprising, in combination:

a. a closed vacuum housing for maintaining a discharge gas of at least one of the heavy hydrogen isotopes under a low pressure;

b. ion generating means including a hollow cylindrical anode electrode enclosing a discharge space and a planar cathode electrode arranged at one of the open frontal faces of said anode electrode completely covering said frontal face of said anode electrode so as to limit said discharge space in the axial direction and the other frontal face of said anode electrode remaining completely open so as to form an emission opening;

c. a first magnetic pole surface of a first polarity disposed in parallel with and at a small distance from the side of said cathode electrode facing away from said discharge space, said first magnetic pole surface covering the cross-sectional area of said anode electrode and being connected therewith, a second annular magnetic pole surface of a second polarity arranged at the edge of said emission opening of said anode electrode, both of said magnetic pole surfaces being connected by said anode electrode;

d. magnetic field means for establishing a magnetic field emanating from said first magnetic pole surface and substantially laterally penetrating said cathode electrode and passing through said discharge space in the direction of the axis of said anode electrode substantially parallel to said anode electrode and being increasingly laterally deflected in the vicinity of said emission opening in the direction toward said second annular magnetic pole surface arranged at the edge of said anode electrode and ending at said second annular magnetic pole surface, said magnetic field means including a current carrying magnetic coil for generating said magnetic field passing through said discharge space, said magnetic coil surrounding the outer surface of said anode electrode. and a magnetic yoke surrounding the outer surface of said magnetic coil and connecting said magnetic pole surfaces together;

e. an acceleration electrode disposed at a short distance from the edge of said emission opening and said second annular magnetic pole surface, said acceleration electrode having an opening corresponding in its width to said emission opening and covering the same and being laterally widened in its portion facing away from said emission opening;

f. a target electrode arranged at the side of said acceleration electrode facing away from said emission opening, said target electrode being connected with said acceleration electrode and having a lateral cross section corresponding to the cross section of an impinging ion stream, said target electrode being provided at its surface facing said ion generating means with an active target layer which contains at least one of the heavy hydrogen isotopes in absorbed form;

g. a current source connected with said magnetic coil, a first high voltage source connected with said cathode electrode and a second high voltage source operatively connected with both said acceleration electrode and said target electrode, the established acceleration potential which is negative with respect to the potential of said anode electrode being substantially higher than the high voltage potential of said cathode electrode which is negative with respect to said anode electrode so that a high voltage low pressure discharge is maintained in said discharge space with the aid of said magnetic field for producing positive ions of the discharge gas which leave said discharge space through said emission opening and are accelerated toward said target electrode by the electric field maintained between said emission opening and said acceleration electrode, thereby causing neutrons to be released from the heavy hydrogen isotopes absorbed in said target layer on said target electrode; and

h. gas pressure control means for storing said discharge gas and for supplying said discharge gas containing at least one of the heavy isotopes of hydrogen to said discharge space so as to regulate the gas pressure within said housing and to replenish the spent gas.

2. A neutron generator as defined in claim ll wherein said anode electrode, said magnetic pole surfaces connected therewith, and portions of said magnetic yoke constitute the covering of said closed vacuum housing.

3. A neutron generator comprising, in combination:

a. a closed vacuum housing for maintaining a discharge gas containing at least one of the heavy hydrogen isotopes under a low pressure;

b. an annular ion generating means including two annular disc-shaped anode electrodes arranged around a common axis and enclosing a discharge chamber and a coaxial annular cathode electrode arranged between said annular anodes so as to surround the outer periphery of said discharge chamber and said discharge chamber having an inner periphery opening towards the axis at a distance therefrom so as to form an annular emission opening, the distance of said emission opening from the axis corresponding to the spacing of said annular anodes from the axis;

0. a first annular magnetic pole surface of a first polarity arranged at the outer periphery of said ion generating means at a slight axial distance from the side of said cathode electrode facing away from said discharge chamber, said first magnetic pole surface being connected with both said anode electrodes, the axial width of said first magnetic pole surface corresponding to the spacing between said anode electrodes, and two second annular magnetic pole surfaces of a second polarity arranged at the inner periphery of said ion generating means and connected with said anode electrodes at the edge of said emission opening, the axial spacing between said second magnetic pole surfaces corresponding to the axial spacing between said anode electrode;

d. magnetic field means for establishing a magnetic field emanating from said first magnetic annular pole surface passing through said cathode electrode, flowing through said annular discharge chamber radially from the outside toward the inside in parallel with said anode electrodes and axially deflected in the area of said emission opening in the direction toward said two second magnetic pole surfaces and toward the inner edges of said anode electrodes, said magnetic field means including two current conducting magnetic coils for generating said magnetic field radially flowing through said annular discharge chamber, said coils being disposed outside of said anode electrodes on the sides facing away from said annular discharge chamber and conduct currents of the same magnitude but directed in opposite directions and two magnetic yokes each enclosing one of said magnetic coils from the outside and connecting said first magnetic pole surface at the outer periphery of said ion generating means with a respective one of said two second magnetic pole surfaces at the edge of said emission opening;

e. two cup-shaped acceleration electrodes coaxially arranged around the axis of said ion generating means in the vicinity of the edges of said emission opening, the axial distance between said acceleration electrodes in the area adjacent to said emission opening corresponding to the width of said emission opening and the space between said acceleration electrodes forming an annular acceleration opening having an axial distance increasing with decreasing radius from the axis;

f. a target electrode having a substantially tubular form and being coaxially arranged about the axis of said ion generating means, said target electrode being disposed between said acceleration electrodes and connected therewith, the axial length of said target electrode corresponding to the axial width of a radially impinging ion stream and said target electrode being provided with an active target layer at the surface facing said ion generating means, said target layer containing in absorbed form at least one of the heavy hydrogen isotopes;

g. a current source connected with said two magnetic coils, a first voltage source connected with said cathode electrode and a second high voltage source operatively connected with both said acceleration electrodes and said target electrode, the potential of said acceleration electrodes which is negative with respect to the potential of said anode electrodes being substantially higher than the high voltage potential of said cathode electrode which is negative with respect to the potential of said anode electrodes, so that a high voltage low pressure discharge is maintained within said annular discharge chamber with the aid of said magnetic field for producing positive ions of the discharge gas which leave said annular discharge chamber through said emission opening and are radially inwardly accelerated toward said tubular target electrode by the electric field maintained between said emission opening and said acceleration electrodes so as to cause neutrons tobe released from the heavy hydrogen isotopes absorbed by said target layer; and

h. a gas pressure control means for storing said discharge gas and for supplying said discharge gas containing at least one of the heavy isotopes of hydrogen to said discharge chamber so as to regulate the gas pressure within said housing and to replenish the spent gas.

4. A neutron generator as defined in claim 3 wherein said anode electrode, said magnetic pole surfaces connected therewith, and portions of said magnetic yoke constitute the covering of said closed vacuum housing.

5. Neutron generator as defined in claim 3, further comprising cooling channels for the passage of a coolant.

6. Neutron generator as defined in claim 3, wherein said magnet coils are enclosed in a watertight shell of corrosion resistant material and are enclosed by said magnet yoke and said anode electrodes so as to form a cooling channel for the passage of a coolant for cooling the coils on the vacuum housing.

7. Neutron generator as defined in claim 3, wherein said cathode electrode is a ring of metal tubes whose windings are welded or soldered to each other.

8. Neutron generator as defined in claim 3, wherein said target electrode is a metal tube supporting structure with a metal tube wound upon said support tube in a bifilar arrangement in the region of the ion current providing a coolant conduit for transfer of a coolant, the windings of said coolant conduit being welded to each other.

9. Neutron generator as defined in claim 3, further comprising a guide tube for guiding an irradiation specimen into the area of said target electrode and being connected with a rabbit system, said guide tube being installed coaxially with the axis of said ion generating means and equipped with penetrations at this end.

10. Neutron generator as defined in claim 3, further comprising a high voltage insulator connected to one side of said ion generating means in the axial direction so as that it is vacuum tight and a support tube passing through the center of said annular ion generating means and said insulator and being attached to its outer front so as to be vacuum tight, and said support tube being extended beyond the point of connection with said insulator and provided with a connection device for the introduction of gas into an internal annular channel.

11. Neutron generator as defined in claim 3, further comprising a guide tube for carrying an irradiation sample into the center of said target electrode and two insulators for attaching said guide tube so as to be vacuum tight on both sides of said target electrode in the axial direction.

12. Neutron generator as defined in claim 3, further comprising a capsule for accommodation of a sample to be irradiated with neutrons, a guide tube for supplying said capsule to the area of said target electrode, means for dispensing said capsule into said guide tube, positioning means for fixing said capsule in a part of said guide tube enclosed by said target electrode for a predetermined period of time for irradiation with neutrons.

13. Neutron generator as defined in claim 3, wherein said tubular target electrode is closed on one end and is designed as a slightly conical frustrum at its closed end and which conical frustrum is part of a cone whose tip is outside said vacuum housing.

14. Neutron generator as defined in claim 3, further comprising collimation means for collimating the neutrons flux emitted from said target layer connected to said vacuum housing on the side provided for the emission of neutrons.

15. Neutron generator as defined in claim 14, wherein said collimation means consist essentially of a collimation cone of a conical frustrum constituted by said target electrode, and a block with a conical or cylindrical bore therein forming a collimation channel, and said collimation cone extends into the inlet section of said collimation channel. 

1. A neutron generator comprising, in combination: a. a closed vacuum housing for maintaining a discharge gas of at least one Of the heavy hydrogen isotopes under a low pressure; b. ion generating means including a hollow cylindrical anode electrode enclosing a discharge space and a planar cathode electrode arranged at one of the open frontal faces of said anode electrode completely covering said frontal face of said anode electrode so as to limit said discharge space in the axial direction and the other frontal face of said anode electrode remaining completely open so as to form an emission opening; c. a first magnetic pole surface of a first polarity disposed in parallel with and at a small distance from the side of said cathode electrode facing away from said discharge space, said first magnetic pole surface covering the cross-sectional area of said anode electrode and being connected therewith, a second annular magnetic pole surface of a second polarity arranged at the edge of said emission opening of said anode electrode, both of said magnetic pole surfaces being connected by said anode electrode; d. magnetic field means for establishing a magnetic field emanating from said first magnetic pole surface and substantially laterally penetrating said cathode electrode and passing through said discharge space in the direction of the axis of said anode electrode substantially parallel to said anode electrode and being increasingly laterally deflected in the vicinity of said emission opening in the direction toward said second annular magnetic pole surface arranged at the edge of said anode electrode and ending at said second annular magnetic pole surface, said magnetic field means including a current carrying magnetic coil for generating said magnetic field passing through said discharge space, said magnetic coil surrounding the outer surface of said anode electrode, and a magnetic yoke surrounding the outer surface of said magnetic coil and connecting said magnetic pole surfaces together; e. an acceleration electrode disposed at a short distance from the edge of said emission opening and said second annular magnetic pole surface, said acceleration electrode having an opening corresponding in its width to said emission opening and covering the same and being laterally widened in its portion facing away from said emission opening; f. a target electrode arranged at the side of said acceleration electrode facing away from said emission opening, said target electrode being connected with said acceleration electrode and having a lateral cross section corresponding to the cross section of an impinging ion stream, said target electrode being provided at its surface facing said ion generating means with an active target layer which contains at least one of the heavy hydrogen isotopes in absorbed form; g. a current source connected with said magnetic coil, a first high voltage source connected with said cathode electrode and a second high voltage source operatively connected with both said acceleration electrode and said target electrode, the established acceleration potential which is negative with respect to the potential of said anode electrode being substantially higher than the high voltage potential of said cathode electrode which is negative with respect to said anode electrode so that a high voltage low pressure discharge is maintained in said discharge space with the aid of said magnetic field for producing positive ions of the discharge gas which leave said discharge space through said emission opening and are accelerated toward said target electrode by the electric field maintained between said emission opening and said acceleration electrode, thereby causing neutrons to be released from the heavy hydrogen isotopes absorbed in said target layer on said target electrode; and h. gas pressure control means for storing said discharge gas and for supplying said discharge gas containing at least one of the heavy isotopes of hydrogen to said discharge space so as to regulate the gas pressure within said housing and to replenish the spent gas.
 2. A neutron generator as defined in claim 1 wherein said anode electrode, said magnetic pole surfaces connected therewith, and portions of said magnetic yoke constitute the covering of said closed vacuum housing.
 3. A neutron generator comprising, in combination: a. a closed vacuum housing for maintaining a discharge gas containing at least one of the heavy hydrogen isotopes under a low pressure; b. an annular ion generating means including two annular disc-shaped anode electrodes arranged around a common axis and enclosing a discharge chamber and a coaxial annular cathode electrode arranged between said annular anodes so as to surround the outer periphery of said discharge chamber and said discharge chamber having an inner periphery opening towards the axis at a distance therefrom so as to form an annular emission opening, the distance of said emission opening from the axis corresponding to the spacing of said annular anodes from the axis; c. a first annular magnetic pole surface of a first polarity arranged at the outer periphery of said ion generating means at a slight axial distance from the side of said cathode electrode facing away from said discharge chamber, said first magnetic pole surface being connected with both said anode electrodes, the axial width of said first magnetic pole surface corresponding to the spacing between said anode electrodes, and two second annular magnetic pole surfaces of a second polarity arranged at the inner periphery of said ion generating means and connected with said anode electrodes at the edge of said emission opening, the axial spacing between said second magnetic pole surfaces corresponding to the axial spacing between said anode electrode; d. magnetic field means for establishing a magnetic field emanating from said first magnetic annular pole surface passing through said cathode electrode, flowing through said annular discharge chamber radially from the outside toward the inside in parallel with said anode electrodes and axially deflected in the area of said emission opening in the direction toward said two second magnetic pole surfaces and toward the inner edges of said anode electrodes, said magnetic field means including two current conducting magnetic coils for generating said magnetic field radially flowing through said annular discharge chamber, said coils being disposed outside of said anode electrodes on the sides facing away from said annular discharge chamber and conduct currents of the same magnitude but directed in opposite directions and two magnetic yokes each enclosing one of said magnetic coils from the outside and connecting said first magnetic pole surface at the outer periphery of said ion generating means with a respective one of said two second magnetic pole surfaces at the edge of said emission opening; e. two cup-shaped acceleration electrodes coaxially arranged around the axis of said ion generating means in the vicinity of the edges of said emission opening, the axial distance between said acceleration electrodes in the area adjacent to said emission opening corresponding to the width of said emission opening and the space between said acceleration electrodes forming an annular acceleration opening having an axial distance increasing with decreasing radius from the axis; f. a target electrode having a substantially tubular form and being coaxially arranged about the axis of said ion generating means, said target electrode being disposed between said acceleration electrodes and connected therewith, the axial length of said target electrode corresponding to the axial width of a radially impinging ion stream and said target electrode being provided with an active target layer at the surface facing said ion generating means, said target layer containing in absorbed form at least one of the heavy hydrogen isotopes; g. a current source connected with said two magnetic coils, a first voltage source connected with said cathode electrode and a second high voltage source operatively connected with both said acceleration electrodes and said target electrode, the potential of said acceleration electrodes which is negative with respect to the potential of said anode electrodes being substantially higher than the high voltage potential of said cathode electrode which is negative with respect to the potential of said anode electrodes, so that a high voltage low pressure discharge is maintained within said annular discharge chamber with the aid of said magnetic field for producing positive ions of the discharge gas which leave said annular discharge chamber through said emission opening and are radially inwardly accelerated toward said tubular target electrode by the electric field maintained between said emission opening and said acceleration electrodes so as to cause neutrons to be released from the heavy hydrogen isotopes absorbed by said target layer; and h. a gas pressure control means for storing said discharge gas and for supplying said discharge gas containing at least one of the heavy isotopes of hydrogen to said discharge chamber so as to regulate the gas pressure within said housing and to replenish the spent gas.
 4. A neutron generator as defined in claim 3 wherein said anode electrode, said magnetic pole surfaces connected therewith, and portions of said magnetic yoke constitute the covering of said closed vacuum housing.
 5. Neutron generator as defined in claim 3, further comprising cooling channels for the passage of a coolant.
 6. Neutron generator as defined in claim 3, wherein said magnet coils are enclosed in a watertight shell of corrosion resistant material and are enclosed by said magnet yoke and said anode electrodes so as to form a cooling channel for the passage of a coolant for cooling the coils on the vacuum housing.
 7. Neutron generator as defined in claim 3, wherein said cathode electrode is a ring of metal tubes whose windings are welded or soldered to each other.
 8. Neutron generator as defined in claim 3, wherein said target electrode is a metal tube supporting structure with a metal tube wound upon said support tube in a bifilar arrangement in the region of the ion current providing a coolant conduit for transfer of a coolant, the windings of said coolant conduit being welded to each other.
 9. Neutron generator as defined in claim 3, further comprising a guide tube for guiding an irradiation specimen into the area of said target electrode and being connected with a rabbit system, said guide tube being installed coaxially with the axis of said ion generating means and equipped with penetrations at this end.
 10. Neutron generator as defined in claim 3, further comprising a high voltage insulator connected to one side of said ion generating means in the axial direction so as that it is vacuum tight and a support tube passing through the center of said annular ion generating means and said insulator and being attached to its outer front so as to be vacuum tight, and said support tube being extended beyond the point of connection with said insulator and provided with a connection device for the introduction of gas into an internal annular channel.
 11. Neutron generator as defined in claim 3, further comprising a guide tube for carrying an irradiation sample into the center of said target electrode and two insulators for attaching said guide tube so as to be vacuum tight on both sides of said target electrode in the axial direction.
 12. Neutron generator as defined in claim 3, further comprising a capsule for accommodation of a sample to be irradiated with neutrons, a guide tube for supplying said capsule to the area of said target electrode, means for dispensing said capsule into said guide tube, positioning means for fixing said capsule in a part of said guide tube enclosed by said target electrode for a predetermined period of time for irradiation with neutrons.
 13. Neutron generator as defined in claim 3, wherein said tubular target electrode is closed on one end and is designed as a slightly conical frustrUm at its closed end and which conical frustrum is part of a cone whose tip is outside said vacuum housing.
 14. Neutron generator as defined in claim 3, further comprising collimation means for collimating the neutrons flux emitted from said target layer connected to said vacuum housing on the side provided for the emission of neutrons.
 15. Neutron generator as defined in claim 14, wherein said collimation means consist essentially of a collimation cone of a conical frustrum constituted by said target electrode, and a block with a conical or cylindrical bore therein forming a collimation channel, and said collimation cone extends into the inlet section of said collimation channel. 